Examples of needle-free injectors are described in U.S. Pat. Nos. 5,599,302; 5,062,830; and 4,790,824. These traditional injectors administer medication as a fine, high velocity jet delivered under sufficient pressure to enable the jet to pass through the skin. The pressure used to deliver the medication is typically greater than approximately 4000 p.s.i. inside the compartment that contains the medicament in the injector. Benefits derived from such pressures, in addition to allowing injection without needles, include the speed of the injection, the dispersion of the injected medicament in the tissue and injection delivery without impact from the resistance by the tissue where the medicament is delivered.
Self-injectors or autoinjectors like the ones disclosed in U.S. Pat. Nos. 4,553,962 and 4,378,015 and PCT Publications WO 95/29720 and WO 97/14455 are constructed to inject medicament at a rate and in a manner similar to hand-operated hypodermic syringes. The self-injectors or autoinjectors have needles that are extended at the time of activation to penetrate the user's skin to deliver medicament through movement of the drug container and related needle. Thus the mechanism that provides the force to deliver the medicament in self-injectors and autoinjectors is also used to extend the needle and the drug container to cause the insertion of the needle through the user's skin. The autoinjectors manufactured, for example by Owen Mumford, thus use very low pressures to inject the medicament, which is injected through a needle in a relatively slow stream. The pressures applied in the medicament-containing compartments of this type of device are very low, reaching a maximum of around 60 p.s.i. and take around 6 seconds to inject 1 mL. These devices do not deliver of the medicament using jet injection, so the medicament is delivered in a bolus at the tip the needle, which typically penetrates the patient by typically at least about 12 mm. When these low pressures and injection rates are used with shorter needles, especially those that penetrate the patient around 5 mm or less, there is a high incidence of leakback of the injected medicament around the needle or through the hole in the tissue created.
Prefilled syringes, such as those presently sold by Becton and Dickinson as the BD Hypak™ are intended for slow speed, manual or autoinjector injections. While prefilled syringes are readily available, the manufacturing techniques employed result in dimensional tolerances that traditionally have been considered too loose for jet injectors since the syringe would need to withstand a very sharp application of an elevated pressures sufficient to jet inject the medicament. Additionally, prefilled syringes include portions shaped to hold the needle and flanges for grasping for injection by hand that result in features that can be susceptible to breakage. Residual stresses that are present in the syringe bodies also increase their fragility, which is one of the reasons they have typically been considered too fragile for use in a jet injector. Thus, jet injectors have typically used more robust cartridges without features intended for handheld use, and which are manufactured with tighter tolerances than typical prefilled syringes.
An injector is needed that can reliable inject medicament to a desired site without a substantial risk of the medicament leaking back out from the patient's skin, at a fast speed substantially without regard to tissue resistance, and preferably being able to use a standard prefilled syringe.